So I'm standing in line at a NY deli. I order a turkey on wheat with lettuce and ask for double meat.

The guy behind me says, "You shouldn't eat meat."

I turn and smile and shrug.

"Seriously," he says, his voice getting louder. "Meat is bad for you."

"I know that might be true," I say. "But I like meat."

That was evidently not the right response.

"A friend turned me on to a vegan diet," he says. "Only fools eat meat. Meat is terrible for you. There's not a single reason to eat meat. The science is irrefutable." Then he paused and moved closer, narrowing his eyes and staring intently into mine. "It's changed my life," he said.

"I'm not sure all meat is bad..." I say, easing back. "But that's really cool how being a vegan has worked out for you. How long have you been doing it?"

"This is my second day," he said, proudly.

Oh.

What happened is a perfect example of the Dunning-Kruger effect. As Dunning says, "If you're incompetent, you can't know you're incompetent... the skills you need to produce a right answer are exactly the skills you need to recognize what a right answer is."

Or, as Bertrand Russell said, "One of the painful things about our time is that those who feel certainty are stupid, and those with any imagination and understanding are filled with doubt and indecision."

Or, as Shakespeare wrote, "The fool doth think he is wise, but the wise man knows himself to be a fool."

Or as my grandfather said, "The dumber you are, the more you think you know."

The D-K effect also extends to evaluating performance. Low-ability individuals tend to assess their ability as much higher than it really is. (There's a flip side: people with high ability tend to underestimate how good they are. High-ability individuals tend to underestimate their relative competence and at the same time assume that tasks which are easy for them are just as easy for other people.)

Of course my deli friend isn't the only only one. I'm often guilty of the same thing. I once spent twenty minutes trying to convince a motorcycle mechanic that my bike was handling poorly because of issues like spring rate and steering head angle and frame height and swingarm stiffness... only to find out that I had unknowingly turned my rear shock's rebound damping to its lowest setting.

By wildly overestimating my knowledge, I became a D-K.

Cocky and conceited people tend to take a position and then proclaim, bluster, and totally disregard differing opinions or points of view. They know they're right -- and they want (actually, they need) you to know it, too. Their behavior isn't a sign of intelligence, though; it's the hallmark of a D-K.

Wisdom isn't found in certainty; wisdom is found in knowing that while you might know a lot, there's also a lot you don't know. Wisdom is found in trying to find out what is right rather than trying to be right. Wisdom is found in realizing when you're wrong and backing down graciously.

D-K people don't do that that.

I should.

And so should you. We shouldn't be afraid to be wrong. We shouldn't be afraid to admit we don't have all the answers. We shouldn't be afraid to say "I think" instead of "I know."

The guy who always thinks he's right? He's a D-K.

The sub-par performer who thinks he's a superstar? He's a D-K.

Or, as we like to say where I'm from, "He's a d - - k."